Lithium ion polymer secondary cell

ABSTRACT

A porous element containing a fluoropolymer comprising vinylidene fluoride as a main unit and having a density of 0.55-1.30 g/cm 3  and a Gurley value of not more than 150 sec/100 cc is used as a polymer substrate of a solid electrolyte to be placed between a positive electrode and a negative electrode. As a result, the solid electrolyte layer shows fine ion conductivity and an ion polymer secondary battery having strikingly improved low temperature characteristics, cycle characteristics and high-rate discharge characteristics as compared to conventional batteries can be obtained.

TECHNICAL FIELD

[0001] The present invention relates to a lithium ion polymer secondarybattery.

BACKGROUND ART

[0002] Lithium ion secondary battery is generally formed by disposing anelectrolyte between a positive electrode and a negative electrode. Thepositive electrode and negative electrode are respectively composed byforming a layer containing an active material, a conductive material, abinder and the like on the surface of a collector. As the activematerial for the positive electrode, Li—Mn type composite oxide, Li—Nitype composite oxide, Li—Co type composite oxide and the like are usedand as the active material for the negative electrode, carbon materialsare used.

[0003] Since lithium ion secondary batteries are capable of achievinghigh energy density and high voltage as compared to Nickel-Cadmiumbatteries and the like, they have been rapidly employed in recent yearsas an operate source of portable equipment, such as portable telephonesand notebook personal computers. An expansion in the applicable range isexpected in the future. In view of this, there have been activelystudied lithium ion secondary batteries aiming for improved batteryperformance.

[0004] For example, what is called lithium ion polymer secondarybatteries, wherein a solid electrolyte layer is disposed between apositive electrode and a negative electrode, have been drawing attentionand being studied recently. The solid electrolyte layer is prepared suchthat a polymer substrate is impregnated with an electrolytic solution(lithium salt (electrolyte)+compatible solvent) which gels to show ionconductivity by itself. When a solid electrolyte layer is used, anelectrolytic solution does not exist in a liquid state (the state whereit flows by itself) within a battery, which in turn affords a hugeadvantage of absence of a leak from the battery. However, the batterycharacteristics (particularly, low temperature characteristics, cyclecharacteristics, high-rate discharge characteristics) of lithium ionsecondary batteries using such a solid electrolyte layer tend to showinferior characteristics as compared to those of liquid type lithium ionsecondary batteries (electrolytic solution+separator such aspolyolefin), and the improvement thereof is a major goal.

[0005] In view of the above-mentioned situation, it is an object of thepresent invention to provide a lithium ion polymer secondary batteryhighly improved in all the low temperature characteristic, cyclecharacteristic and high-rate discharge characteristic.

DISCLOSURE OF THE INVENTION

[0006] The present inventors have conducted intensive studies with theaim of achieving the above-mentioned object, and as a result, have foundthat, by using a porous element of a fluoropolymer containing vinylidenefluoride as a main unit as a solid electrolyte polymer substrate, thebattery characteristics can be improved and that the batterycharacteristics (particularly, low temperature characteristics, cyclecharacteristics and high-rate discharge characteristics) can be markedlyimproved by the use of one having a particular density and Gurley value,because the density and Gurley value of the porous element greatlyaffect the battery characteristics, which resulted in the completion ofthe present invention.

[0007] Accordingly, the lithium ion polymer secondary battery of thepresent invention is characterized by the following.

[0008] (1) A lithium ion polymer secondary battery comprising a positiveelectrode, a negative electrode and a solid electrolyte layer comprisinga porous element comprising a fluoropolymer comprising vinylidenefluoride as a main unit and having a density of 0.55-1.30 g/cm³ and aGurley value of not more than 150 sec/100 cc, a salt and a compatiblesolvent, which is disposed between the positive electrode and thenegative electrode.

[0009] (2) The lithium ion polymer secondary battery of theabove-mentioned (1), wherein the salt is at least one kind of compoundselected from LiClO₄, LiBF₄, LiPF₆, LiAsF₆, LiCF₃SO₃, LiAlCl₄ andLi(CF₃SO₂)₂N, and the above-mentioned compatible solvent is a mixedsolvent of one or more kinds selected from ethylene carbonate, propylenecarbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate,dimethyl sulfoxide, sulfolane, y-butyrolactone, 1,2-dimethoxyethane,N,N-dimethylformamide, tetrahydrofuran, 1,3-dioxolane,2-methyltetrahydrofuran and diethyl ether.

[0010] (3) The lithium ion polymer secondary battery of theabove-mentioned (1) or (2), wherein the positive electrode activematerial is a Li-transition metal composite oxide.

[0011] (4) The lithium ion polymer secondary battery of any of theabove-mentioned (1) to (3), wherein the negative electrode activematerial is a graphite.

[0012] (5) The lithium ion polymer secondary battery of any of theabove-mentioned (1) to (4), wherein

[0013] the positive electrode is a belt-shaped positive electrodecomprising positive electrode active material layers, which comprise anactive material and a conductive material, formed on both surfaces of abelt-shaped collector,

[0014] the negative electrode is a belt-shaped negative electrodecomprising negative electrode active material layers formed on bothsurfaces of a belt-shaped collector, and

[0015] these belt-shaped positive electrode and belt-shaped negativeelectrode and the solid electrolyte layer having a belt shape and beinginterposed between said electrodes are spirally wound to constitute aroll, wherein

[0016] the total thickness A of the positive electrode active materiallayers formed on both surfaces of the belt-shaped collector of theabove-mentioned belt-shaped positive electrode and the total thickness Bof the negative electrode active material layers formed on both surfacesof the belt-shaped collector of the above-mentioned belt-shaped negativeelectrode are each 80 μm-250 μm, and

[0017] the ratio (A/B) of the total thickness A to the total thickness Bis 0.4-2.2.

[0018] (6) The lithium ion polymer secondary battery of any of the.above-mentioned (1) to (4), wherein

[0019] the positive electrode is a belt-shaped positive electrodecomprising positive electrode active material layers, which comprise anactive material and a conductive material, formed on both surfaces of abelt-shaped collector,

[0020] the negative electrode is a belt-shaped negative electrodecomprising negative electrode active material layers formed on bothsurfaces of a belt-shaped collector, and

[0021] these belt-shaped positive electrode and belt-shaped negativeelectrode and the solid electrolyte layer having a belt shape and beinginterposed between the electrodes are spirally wound to constitute aroll, wherein

[0022] an outermost roll part of the above-mentioned belt-shapednegative electrode is disposed on a still outer periphery of theoutermost roll part of the above-mentioned belt-shaped positiveelectrode, and a first extrusion part extruding from a free end of theoutermost roll part of the above-mentioned belt-shaped positiveelectrode is formed on a free end of the outermost roll part of theabove-mentioned belt-shaped negative electrode,

[0023] an innermost roll part of the above-mentioned belt-shapednegative electrode is disposed on a still inner periphery of theinnermost roll part of the above-mentioned belt-shaped positiveelectrode and a second extrusion part extruding from a free end of theinnermost roll part of the above-mentioned belt-shaped positiveelectrode is formed on a free end of the innermost roll part of theabove-mentioned belt-shaped negative electrode, and

[0024] a third and a fourth extrusion parts extruding from both ends inthe width direction of the above-mentioned belt-shaped positiveelectrode are respectively formed on both ends in the width direction ofthe above-mentioned belt-shaped negative electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 shows a cross sectional view of one embodiment of the powergeneration element (roll) of the lithium ion polymer secondary batteryof the present invention, comprising a positive electrode, a negativeelectrode and a solid electrolyte layer.

[0026]FIG. 2 shows the size and the positional relationship between thebelt-shaped positive electrode and the belt-shaped negative electrodeconstituting the roll shown in FIG. 1.

[0027]FIG. 3 shows a cross sectional view of another embodiment of thepower generation element of the lithium ion polymer secondary battery ofthe present invention, comprising a positive electrode, a negativeelectrode and a solid electrolyte layer.

[0028] In FIG. 1 and FIG. 2, 1 shows a belt-shaped positive electrode, 1a shows an outermost roll part of the belt-shaped positive electrode, 1b shows an innermost roll part of the belt-shaped positive electrode, 2shows a belt-shaped negative electrode, 2 a shows an outermost roll partof the belt-shaped negative electrode, 2 b shows an innermost roll partof the belt-shaped negative electrode, 2A shows a first extrusion part,2B shows a second extrusion part, 2C-1 shows a third extrusion part,2C-2 shows a fourth extrusion part, 3 shows a solid electrolyte layer,and 100 shows a roll. In FIG. 3, 11 shows a rectangular negativeelectrode plate, 12 shows a belt-shaped member of the solid electrolytelayer, and 13 is a rectangular positive electrode plate.

DETAILED DESCRIPTION OF THE INVENTION

[0029] The lithium ion polymer secondary battery of the presentinvention is characterized in that a porous solid electrolyte layercomprising a porous element comprising a fluoropolymer comprisingvinylidene fluoride as a main unit and having a density of 0.55-1.30g/cm³ and a Gurley value of not more than 150 sec/100 cc is disposed, asa polymer substrate, between a positive electrode and a negativeelectrode. Such constitution realizes fine low temperaturecharacteristics, cycle characteristics and high-rate dischargecharacteristics.

[0030] As used herein, the density of a porous element. comprising afluoropolymer comprising vinylidene fluoride as a main unit means an“apparent density”.

[0031] In the present invention, by the “fluoropolymer comprisingvinylidene fluoride as a main unit” is meant a homopolymer of vinylidenefluoride (polyvinylidene fluoride (PVdF)), copolymers of vinylidenefluoride and other vinyl monomer having fluorine atom, wherein theproportion of the vinylidene fluoride (unit) is at least 70 mol %, andthese polymers further grafted with vinyl polymers containing thebelow-mentioned functional groups. These may be used alone or two ormore thereof may be used in combination. The above-mentioned other vinylmonomer having fluorine atom, which is other than vinylidene fluoride,is exemplified by hexafluoropropylene (HFP), chlorotrifluoroethylene(CTFE), tetrafluoroethylene (TFE) and the like. The form of theabove-mentioned copolymer may be random or block. The proportion ofvinylidene fluoride (unit) in the above-mentioned copolymer ispreferably not less than 75 mol %.

[0032] The functional group-containing vinyl polymer that grafts theabove-mentioned homopolymer of vinylidene fluoride or the copolymer ofvinylidene fluoride and other vinyl monomer having fluorine atom isexemplified by polymers of, vinyl monomer having a functional group suchas carboxyl group (—COOH), sulfo group (—SO₂OH), carboxylic acid estergroup (—COOR), amide group (—CONH₂), phosphoric acid group (—PO(OH)₂)and the like. As used herein, the substituent R of the carboxylic acidester group (—COOR) is a lower alkyl group having 1 to 4 carbon atoms,such as methyl group, ethyl group, butyl group and the like. In the caseof the form of polymer wherein such functional group-containing vinylpolymer is grafted, the adhesive property of the solid electrolyte layerto the positive electrode or negative electrode is improved, preferablyfurther reducing the resistance between electrodes. The vinyl monomerhaving the above-mentioned functional group is preferably a vinylcompound (monomer) wherein the moiety other than the functional grouphas not more than 4 carbon atoms. As the carboxyl group-containingmonomer, those having one carboxyl group such as acrylic acid,methacrylic acid, crotonic acid, vinyl acetate, allyl acetate and thelike, as well as those having two carboxyl groups such as itaconic acid,maleic acid and the like can be used. As the sulfo group-containingmonomer, styrene sulfonate, vinyl sulfonate and the like are preferable.As the carboxylic acid ester group-containing monomer, methyl acrylate,butyl acrylate and the like are preferable. As the amid group-containingmonomer, acrylamide and the like are preferable. As the phosphoric acidgroup-containing monomer, triphenyl phosphate, tricresyl phosphate andthe like are preferable. Of these, most preferred are acrylic acid andmethacrylic acid.

[0033] The method of grafting is preferably a radiation method. Forexample, polymer chain substrate (polymer to be grafted) and a graftmonomer material are co-existed and radiation is continuously orintermittently applied. More preferably, the polymer substrate isirradiated in advance before co-using the both. For radiation, electronbeam, X-rays or γ-rays are used. By the irradiation, the polymersubstrate generates a free group and is activated.

[0034] The degree of grafting can be determined according to somefactors. The most important factors are the length of time of contact ofactivated substrate with a graft monomer, the degree of preliminaryactivation of substrate by irradiation, the degree to allow monomermaterial to permeate the substrate, and the temperature during contactbetween the substrate and the monomer. For example, when the graftmonomer is an acid, a solution containing the monomer is sampled,titrated against base, and remaining monomer concentration is measured,whereby the degree of grafting can be monitored. The degree of graftingis preferably 2-20%, particularly preferably 3-12%, specificallypreferably 5-10%, of the final weight.

[0035] The grafting may be conducted by a method comprising activation(occurrence of free group) by exposure of the polymer substrate to lightor heat.

[0036] In the present invention, a porous element comprising afluoropolymer comprising vinylidene fluoride as a main unit and having aparticular density (0.55-1.30 g/cm³) and a particular Gurley value (nothigher than 150 sec/100 cc) is used as a polymer substrate of the solidelectrolyte layer, which greatly improves the battery characteristics.This is considered to be attributable to the fact that a polymersubstrate is present in a solid electrolyte at a suitable proportion,and the continuous bubble (pore) structure of the polymer substrate isnoticeable, which in turn results in preferable gel state of a solidelectrolyte layer due to ion conduction.

[0037] When the porous element comprising a fluoropolymer comprisingvinylidene fluoride as a main unit has a density of less than 0.55g/cm³, ion conductivity becomes lower because a solid electrolyte layerusing such porous element has a greater portion of liquid phaseinvolving no (less) polymer chain in the layer. As a result, the batterycharacteristics (particularly low temperature characteristics) aredegraded, causing a short circuit, to possibly make charge and dischargeunattainable. In addition, due to degraded mechanical strength of theporous element, handling of the element during the assembly of thebattery may become difficult. In contrast, when the porous element has adensity exceeding 1.30 g/cm³, the improving effect of batterycharacteristics (low temperature characteristics, cycle characteristicsand high-rate discharge characteristics) may be difficult to achieve.

[0038] In the present invention, the porous element comprising afluoropolymer comprising vinylidene fluoride as a main unit preferablyhas a density of 0.60-1.20 g/cm³, more preferably 0.65-0.85 g/cm³.

[0039] In addition, when a porous element comprising a fluoropolymercomprising vinylidene fluoride as a main unit and having a Gurley valueexceeding 150 sec/100 cc, a solid electrolyte layer using such a porouselement does not show fine ion conductivity, and battery characteristics(particularly low temperature characteristics) are degraded even if theporous element has a density within the range of 0.55-1.30 g/cm³. In thepresent invention, the porous element comprising a fluoropolymercomprising vinylidene fluoride as a main unit has a Gurley value ofpreferably not more than 100 sec/100 cc, more preferably not more than50 sec/100 cc. While the lower limit of the Gurley value is notparticularly limited, it is preferably not less than 2 sec/100 cc, morepreferably not less than 5 sec/100 cc.

[0040] In the present invention, the average pore size of the porouselement comprising a fluoropolymer comprising vinylidene fluoride as amain unit is preferably 0.01-10 μm, particularly preferably 0.1-5 μm.This “average pore size” is an average value of pore sizes of optional10 pores as measured by observation with SEM. While it varies dependingon the density and Gurley value of the porous element, when the averagepore size of the pores is less than 0.01 μm, liquid retention capabilitybecomes low, whereas when it exceeds 10 μm, the porous element has alower mechanical strength, which in turn sometimes show lower handlingproperty during the production of batteries.

[0041] In the present invention, the fluoropolymer comprising vinylidenefluoride as a main unit preferably shows a melt flow index at 230° C.,10 kg of not more than 1.0 g/10 min, more preferably 0.2-0.7 g/10 min.When the melt flow index is not more than 1.0 g/10 min, mechanicalstrength and ion conductivity of solid electrolyte layer atroomtemperature are further improved. The melt flow index here ismeasured according to the method defined in standard ASTM D 1238.

[0042] In the present invention, the solid electrolyte layer is formedby impregnating a porous element comprising the above-mentionedfluoropolymer comprising vinylidene fluoride as a main unit with anelectrolytic solution comprising a salt (lithium salt) and a compatiblesolvent. As used herein, by the “compatible solvent” is meant a solventthat dissolves a salt (lithium salt), and dissolves or swells theabove-mentioned fluoropolymer comprising vinylidene fluoride as a mainunit.

[0043] The salt (lithium salt) is exemplified by one or more kindsselected from the group consisting of LiClO₄, LiBF₄, LiPF₆, LiAsF₆,LiCF₃SO₃, LiAlCl₄ and Li(CF₃SO₂)₂N. Of these, LiPF₆ is preferable.

[0044] As the compatible solvent, for example, ethylene carbonate,propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethylcarbonate, dimethyl sulfoxide, sulfolane, γ-butyrolactone,1,2-dimethoxyethane, N,N-dimethylformamide, tetrahydrofuran,1,3-dioxolane, 2-methyltetrahydrofuran, diethyl ether and the like canbe used. These may be used alone or in a mixed solvent comprising two ormore kinds thereof. A mixed solvent of 4 kinds of ethylene carbonate,propylene carbonate, ethylmethyl carbonate and diethyl carbonate ispreferable. In the mixed solvent, ethylene carbonate is preferablycontained in 5-30 vol %, more preferably 15-25 vol %; propylenecarbonate is preferably contained in 5-25 vol %, more preferably 8-20vol %; ethylmethyl carbonate is preferably contained in 40-75 vol %,more preferably 55-65 vol %; and diethyl carbonate is preferablycontained in 5-20 vol %, more preferably 8-15 vol %.

[0045] The salt concentration of the electrolytic solution(salt+compatible solvent) is preferably 0.5-1.5 mol/L, more preferably0.7-1.3 mol/L, particularly preferably 0.8-1.2 mol/L. When theconcentration is less than 0.5 mol/L, ion conductivity is degraded,possibly resulting in insufficient battery capacity or degradedhigh-rate discharge characteristics, whereas when it exceeds 1.5 mol/L,high-rate discharge characteristics and low temperature characteristicsunpreferably tend to be degraded due to higher viscosity.

[0046] It is possible to use plasticizers such as tetraethylene glycoldimethyl ether, N-methyl-pyrrolidone(1-methyl-2-pyrrolidone), ethyleneglycol dimethyl ether, diethylene glycol dimethyl ether and the like,together with the above-mentioned compatible solvent. The use of saidplasticizer preferably prevents crystallization of electrolytic solution(salt+compatible solvent) during use of the battery at low temperature(particularly not higher than −10° C.). The amount of the plasticizer tobe used is preferably about 1-50 wt % of the compatible solvent.

[0047] In the present invention, the thickness of the solid electrolytelayer varies depending on the shape, size and the like of the positiveelectrode and negative electrode. In general, the average thickness ispreferably 5-100 μm, particularly preferably 8-50 μm, specificallypreferably 10-30 μm. As used herein, the thickness of the solidelectrolyte layer refers to the thickness when it is disposed betweenthe positive electrode and the negative electrode (when battery isactually assembled) and equals to the distance between the positiveelectrode and the negative electrode.

[0048] The method for preparing the solid electrolyte layer in thepresent invention is not particularly limited, and may be, for example,the methods of the following (a)-(c) and the like.

[0049] (a) A method wherein a fluoropolymer comprising vinylidenefluoride as a main unit is formed into a film by a known foam-moldingmethod by extrusion-foaming-molding and the like to give a porous film,or a coating liquid (paste) containing a fluoropolymer, a suitablesolvent and a foaming agent in admixture is prepared, the coating liquid(paste) is applied to the surface of a release substrate with a suitablecoater to form a coating film, the coating film is heated, dried andreleased from the release substrate to give a porous film, and theobtained porous film is immersed in a solution obtained by dissolving asalt in a compatible solvent to allow gellation (including immersion ina solution together with a positive electrode and a negative electrodeduring process for producing a battery).

[0050] (b) A method wherein a salt, a compatible solvent and a foamingagent are dissolved in a suitable solvent, a fluoropolymer is added anddissolved by heating as necessary to give a coating liquid (paste),which is applied to a surface of a release substrate with a suitablecoater to form a coating film, the coating film is heated while raisingthe temperature serially and dried to evaporate the aforementionedsolvent and to simultaneously produce bubbles and the solid electrolytelayer is released from a release substrate. For the above-mentionedheating and drying, a release substrate having a coating film formedthereon may be passed through heating chambers having differenttemperatures.

[0051] (c) a coating film is directly formed with the above-mentionedcoating liquid (paste) containing a salt, a compatible solvent, afoaming agent and a fluoropolymer dissolved therein on at least onesurface of a positive electrode and/or a negative electrode formed in abelt shape, a plate shape and the like, and a solid electrolyte layer isformed by evaporation of solvent and generation of bubbles.

[0052] As the foaming agent to be used for the above-mentioned methods(a)-(c), decomposition foaming agent, gas foaming agent and volatilefoaming agent can be used. For the above-mentioned method (a), a gasfoaming agent or a volatile foaming agent is preferably used. As the gasfoaming agent, nitrogen, carbon dioxide, propane, neopentane,methylether, dichlorodifluoromethane, n-butane, iso-butane and the likeare preferable and as the volatility foaming agent, n-octanol,1-pentanol, 3-methyl-1-butanol, 2-methyl-1-butanol, 2-pentanol,3-pentanol, 2-methyl-2-butanol, 3-methyl-2-butanol and the like arepreferable. For the above-mentioned methods (b) and

[0053] (c), a volatility foaming agent is preferable, and n-octanol,1-pentanol, 3-methyl-1-butanol and the like are particularly preferable,specifically n-octanol is preferable. As the solvent for theabove-mentioned methods (a)-(c), for example, tetrah ydrofuran (THF),dimethylacetamide, dimethylformamide (DMF) and the like are used.

[0054] The density of the porous element is controlled by changing theamount of foaming agents and various conditions of the production. Forexample, in the case of method (a), the production conditions thatmarkedly affect the density (expansion degree) besides the amount of thefoaming agent are molding temperature, molding rate, molding pressureand the like. In the case of methods (b) and (c), the productionconditions that markedly affect the density (expansion degree) besidesthe amount of the foaming agent are coating rate, drying temperatureprofile, degree of exhaustion, molding rate and the like.

[0055] In addition, the Gurley value of the porous element is controlledby, for example, concentration of solvent and foaming agent in a coatingliquid wherein a foaming agent, a fluoropolymer and the like aredissolved, exhaust conditions and application rate during application ofcoating liquid, temperature distribution in the drying furnace duringdrying of coating film, and the like.

[0056] Of the above-mentioned methods (a)-(c), method (a) is preferablein view of easy control of density and Gurley value.

[0057] The active material for the positive electrode of the lithium ionpolymer secondary battery of the present invention is preferablyLi-transition metal composite oxide, and the Li-transition metalcomposite oxide shown by the following formula (I) or (II) isparticularly preferable.

Li_(A)M_(1−X)Me_(X)O₂(I)

[0058] wherein M is a transition metal such as Co, Ni, Mn, V, Ge and thelike, and

Li_(A)M_(2−X)Me_(X)O₄(II)

[0059] wherein M is a transition metal such as Mn, Fe, Ni and the like.In the formulas (I) and (II), Me is a 3-10 group element in the periodictable, such as Zr, V, Cr, Mo, Fe, Co, Mn, Ni and the like, or a 13-15group element, such as B, Al, Ge, Pb, Sn, Sb and the like, provided thatMe and M are different elements and Me may consist of two or more kindsof elements.

[0060] A in the formula (I) is 0.05-1.5, preferably 0.1-1.1, and in theformula (II), it is 0.05-2.5, preferably 0.5-1.5. X in the formula (I)and (II) is 0 or 0.01-0.5, preferably 0.02-0.2. When Me consists of twoor more kinds of elements, X is the total number of the two or morekinds of elements.

[0061] Specific examples of the Li-transition metal composite oxide ofthe formulas (I) and (II) include LiCoO₂, LiNiO₂, LiMnO₂, LiMn_((1−X))Al_(X) O₂, LiMn_((1−X)) Co_(X)O₂, LiMn_((1−X-Y)) Al_(X) Co_(Y) O₂, LiMn₂O₄, LiMn_(2−X) Co_(X) O₄, LiMn_((2−X-Y)) Co_(X) Ge_(Y) O₄, LiCo_((1−X))Ni_(X) O₂, LiNi_((1−X)) Al_(X) O₂, LiCo_((1−X)) Mn_(X) O₂ and the like,wherein 0.1≧X and Y≦0.1, with preference given to LiCoO₂ and LiNiO₂,particularly preferably LiCoO₂.

[0062] The Li-transition metal composite oxide represented by theformula (I) or the formula (II) is preferable particles satisfying thefollowing formula (III).

7≦[20/(specific surface area [m²/g]×average particle size[μm])]≦9  (III)

[0063] Of those satisfying the formula (III), one having an averageparticle size of not less than 10 μm, preferably not less than 17 μm, ismore preferable in view of the safety of batteries. When the averageparticle size exceeds 25 μm, the resistant component of the activematerial markedly increases. Thus, the average particle size ispreferably not more than 25 μm, more preferably not more than 23 μm.

[0064] The average particle size of the above-mentioned Li-transitionmetal composite oxide is measured by the following method.

[0065] First, the particles to be the measurement target are cast in anorganic liquid such as water, ethanol and the like, and dispersed byultrasonication at about 35 kHz-40 kHz for about 2 minutes. Theparticles are in such an amount that makes the laser transmittance(quantity of outgoing light/quantity of incident light) of thedispersion after dispersing treatment 70% -95%. Then, the dispersion issubjected to a microtrack particle size analyzer and the particle size(D1, D2, D3 . . . ) of each particle, and the number (N1, N2, N3 . . . )of particles having each particle size are measured based on thediffusion of a laser beam.

[0066] The microtrack particle size analyzer calculates the particlesize distribution of a spherical particle group having a theoreticalstrength closest to the observed diffusion strength distribution. Thatis, particles are assumed to have a circular section of the same area asthe projected image obtained by the irradiation of a laser beam, and thediameter of the sectional circle is taken as the particle size.

[0067] The average particle size (μm) is calculated from the followingformula (IV) using the particle size (D) of each particle and the number(N) of particles having each particle size, which are obtained above.

average particle size (μm)=(ΣND³/ΣN)^(1/3)  (IV)

[0068] The above-mentioned Li-transition metal composite oxide has aspecific surface area measured by the gas phase adsorption method(single-point method) wherein nitrogen is an adsorbate, from among theadsorption methods described in Material Chemistry of Fine Particles,Yasuo Arai, first edition, 9th impression, Baifukan (Tokyo), pp. 178-184(1995).

[0069] For example, the Li-transition metal composite oxide representedby the formula (I) or (II) can be produced by the following methods.

[0070] One method therefor comprises mixing a starting lithium compoundand a desired transition metal compound to make the atomic ratio of thetransition metal to lithium 1:1-0.8:1, heating the mixture at atemperature of from 700° C. to 1200° C. in the atmosphere for 3 hours-50hours to allow reaction, pulverizing the reaction product into particlesand harvesting those having the objective particle size.

[0071] A different method further includes heating the above-harvestedparticles. The heat treatment of the particles includes heating at about400° C.-750° C., preferably about 450° C.-700° C., for about 0.5 hr.-50hrs., preferably about 1 hr.-20 hrs. When the particles are heattreated, only the specific surface area can be reduced without changingthe average size of the particles. As a result, a Li-transition metalcomposite oxide that satisfies the particular relationship (relationshipof formula (III)) between the above-mentioned particle size and specificsurface area can be easily obtained.

[0072] The atmosphere of the heat treatment of the pulverized particlesis not limited, and may be the air or an inert gas (e.g., nitrogen,argon) atmosphere. When carbonic acid gas.is present in the atmosphere,however, lithium carbonate is generated and the content of the impuritymay increase. Thus, the heat treatment is preferably conducted in anatmosphere having a carbonic acid gas partial pressure of not more thanabout 10 mmHg.

[0073] The lithium compound to be a starting material is exemplified bylithium oxide, lithium hydroxide, lithium halide, lithium nitrate,lithium oxalate, lithium carbonate and mixtures thereof. Examples of thetransition metal compound include oxide of transition metal, hydroxideof transition metal, halide of transition metal, nitrate of transitionmetal, oxalate of transition metal, carbonate of transition metal, andmixtures thereof. When the desired composite oxide contains thesubstituted element of (Me), a necessary amount of a compound of thesubstituted element of (Me) is added to a mixture of a lithium compoundand a transition metal compound.

[0074] In the lithium ion secondary battery of the present invention,the positive electrode generally consists of a composition layer(hereinafter to be referred to as a positive electrode active materiallayer) containing at least an active material, a conductive material anda binder. The positive electrode is generally formed into a rectangularplate, a belt-shaped member and the like, which is used in 1 (sheet), orplural (sheets), per one battery according to the objective batterycapacity. In the case of 1 (sheet), or plural (sheets of) positiveelectrodes, a positive electrode active material layer may be formed onone or both surface(s) of a collector.

[0075] As the conductive material, particulate carbon materials,conventionally used for conductive material for positive electrode oflithium ion secondary battery are used. Here, the “particulate” includesscaly, spherical, pseudospherical, bulky, whisker and the like, wherein2 or more kinds of particles having a different shapes may be present.The particulate carbon material is exemplified by carbon blacks such asartificial or natural graphites (graphite carbon), KETJENBLACK,acetylene black, oil furnace black, extraconductive furnace black andthe like. These may be used alone or two or more thereof may be used incombination.

[0076] Preferable embodiments of the conductive material includegraphites having an average particle size of 3-8 μm (preferably 4-7 μm),and carbon blacks having an average particle size of not more than 0.1μm (preferably not more than 0.01 μm) at a weight ratio(graphites:carbon blacks) of 1:0.01-1:1, preferably 1:0.1-1:0.5. Whilethe lower limit of the average particle size of carbon blacks is notparticularly limited, it is preferable not less than 0.005 μm. In thisembodiment, the clearance between the active material (particles) andthe active material (particles) is mainly embedded by graphites having agreater particle size, and carbon blacks having a smaller particle sizemainly cover the surface of the active material, thereby ensuring thehigh conductivity of the positive electrode, and improving the lowtemperature characteristics, cycle characteristics and high-ratedischarge characteristics of the battery. In this embodiment, thegraphite is preferably graphitized carbon showing a spacing of latticeplanes (d002) of not more than 0.34 nm and crystallite size in thec-axis direction (Lc) of not less than 10 nm, and carbon black ispreferably oil furnace black.

[0077] The particle size of the conductive material (particulate carbonmaterial) is meant a diameter of section assuming particles to bespheres (diameter corresponding to sphere), and can be measured in thesame manner as in the aforementioned Li-transition metal composite oxideparticles using a microtrack particle size analyzer. The spacing oflattice planes (d002) and crystallite size in the c-axis direction (Lc)of the above-mentioned graphites can be measured according to JapanSociety for the Promotion of Science Method. The method is explained indetail in the following.

[0078] First, highly pure silicon for X-ray standard is pulverized tonot more than 325 mesh standard sieve in an agate mortar to give astandard substance. The standard substance and graphitized carbon, whichis a specimen to be measured, are mixed in an agate mortar (standardsubstance 10 wt % relative to graphitized carbon 100 wt %) to give aspecimen for X-ray. This specimen for X-ray is uniformly filled in asample board for an X-ray diffraction apparatus RINT2000 (RIGAKUELECTRIC CORPORATION, ray source: CuKα ray). Then, setting the voltageapplied on an X-ray tube for 40 kV, the current to be applied for 50 mA,scanning range to 2θ=23.5 degree −29.5 degree, and scanning speed for0.25 degree/min, the 002 peak of carbon and 111 peak of the standardsubstance are measured. Then, using the graphitized degree calculationsoft attached to the above-mentioned X-ray diffraction apparatus, thespacing of lattice planes (d002) and crystallite size in the c-axisdirection (Lc) are calculated from the obtained peak position and thehalf-width.

[0079] The amount of the conductive material to be used is generallyabout 3-15 parts by weight, preferably 3-10 parts by weight, relative to100 parts by weight of the active material.

[0080] As the binder, polyvinylidene fluoride, polytetrafluordethylene,polyethylene, ethylene-propylene-diene type polymer and the like arepreferably used. The amount of the binder to be used is preferably 1-20parts by weight, more preferably 2-10 parts by weight, per 100 parts byweight of the active material.

[0081] As the collector on which to form a positive electrode activematerial layer, for example, those similar to conventional ones such asfoil, expanded metal and the like formed from aluminum, aluminum alloy,titanium and the like can be used.

[0082] The positive electrode active material layer can be generallyformed by (1) a kneading step, (2) a coating step, (3) a drying step and(4) a roll-spreading step.

[0083] In the kneading step (1), the aforementioned active material,conductive material, binder and the like are kneaded in conventionallyknown N-methylpyrrolidone using, for example, a conventionally knownkneading apparatus such as planetary dispa kneading apparatus (ASADAIRON WORKS CORPORATION) and the like in the manner generally employed inthis field to achieve uniform dispersion to give a slurry.

[0084] In the subsequent coating step (2), the above-mentioned obtainedslurry is applied on a collector as generally done in this field using aconventionally known application apparatus such as a commarole type ordiecoat type application apparatus and the like.

[0085] In the drying step (3), the slurry applied to the collector isdried in a warm air drying oven and the like in, for example, atemperature range of 100-200° C. for 5-20 min.

[0086] In the subsequent roll-spreading step (4), using an apparatussuch as rolling press apparatus and the like, the above-mentioned slurrydried on a collector is roll-spread in layers to form a positiveelectrode active material layer. The roll-spreading conditions of thisroll-spreading step, i.e., roll-spreading temperature and roll-spreadingrate, are set for particular ranges to control porosity of the formedpositive electrode active material layer.

[0087] The roll-spreading temperature in the rolling step is preferably20-100° C., more preferably 25-50° C., and the roll-spreading rate ispreferably 10-40%, more preferably 20-35%. When both the roll-spreadingtemperature and roll-spreading rate are less than the above-mentionedranges, spring back occurs due to the roll-spreading at a lowtemperature and the safety of the obtained lithium ion secondary batterydecreases. In addition, due to low roll-spreading rate, inconveniencesmay arise, such as a failure to achieve the design capacity, loweradhesion between the positive electrode active material layer and thecollector. When both the roll-spreading temperature and roll-spreadingrate exceed the above-mentioned ranges, due to a high temperatureroll-spreading and the high-rate discharge characteristics tend to bedegraded. When the roll-spreading rate is within the above-mentionedrange and the roll-spreading temperature is less than theabove-mentioned range, the design capacity may be achieved but due tothe spring back, the safety of the obtained lithium ion secondarybattery may be degraded. When the roll-spreading rate is within theabove-mentioned range and the roll-spreading temperature exceeds theabove-mentioned range, the design capacity may be achieved but due toinsufficient immersion of an electrolytic solution, the resistance ofthe electrode may become higher. Furthermore, when the roll-spreadingtemperature is within the above-mentioned range and the roll-spreadingrate is less than the above-mentioned range, the roll-spreading cannotbe applied sufficiently, which in turn may result in the degradation ofthe cycle characteristics due to lower adhesion between the positiveelectrode active material layer and the collector. When theroll-spreading temperature is within the above-mentioned range and theroll-spreading rate exceeds the above-mentioned range, the high-ratedischarge characteristics may be degraded. As used herein, theroll-spreading temperature means the temperature during processing ofthe material used for roll-spreading processing and the roll-spreadingrate means a parameter expressing the roll-spreading processing degreealso called draught rate and the like. The roll-spreading rate iscalculated according to the following formula (V), wherein hl is thethickness before roll-spreading, h2 is the thickness afterroll-spreading, and h3 is the thickness of the collector.

roll-spreading rate (%)=(h1-h2)×100/(h1-h3)  (V)

[0088] The thickness of the positive electrode active material layer ispreferably 80-300 μm, particularly preferably 80-250 μm, specificallypreferably 120-160 μm. As used herein, the “thickness of the positiveelectrode active material layer” refers to the thickness of the positiveelectrode active material layer formed on one surface of a collector,when the positive electrode active material layer is formed on onesurface thereof, and when the active material layers are formed on bothsurfaces of the collector, the total of the thickness of the twopositive electrode active material layers formed on the both surfacesthereof. When the thickness of the positive electrode active materiallayer is less than 80 μm, the insufficient application amount causeslower charge and discharge capacity and possible unpreferabledegradation of the high-rate discharge characteristics and lowtemperature characteristics due to excessive roll-spreading. When thethickness exceeds 300 μm, the adhesion between the active material layerand the collector decreases markedly to possibly result in degradedcycle characteristics. Particularly when the positive electrode and thenegative electrode to be mentioned later are to be belt-shaped, and aroll is composed by spirally winding upon disposing a belt-shaped solidelectrolyte between the belt-shaped positive electrode and thebelt-shaped negative electrode, which roll is then housed in an exteriorpackage (battery can etc.) to give a battery, the outer diameter of theroll exceeds the design value, which may inconveniently prevent easyinsertion of the roll into the exterior package (battery can etc.).

[0089] In the lithium ion secondary battery of the present invention, anegative electrode generally is composed by forming a composition layer(hereinafter to be referred to as a negative electrode active materiallayer) containing at least a negative electrode active material and abinder on a collector. Like the positive electrode, the negativeelectrode is generally formed into a rectangular plate, a belt-shapedmember and the like, which is used in 1 (sheet), or plural (sheets), perone battery according to the objective battery capacity. In the case of1 (sheet), or plural (sheets of) negative electrodes, a negativeelectrode active material layer may be formed on one or both surface(s)of a collector.

[0090] The negative electrode active material is not particularlylimited and carbon materials conventionally used as negative electrodeactive materials of lithium ion secondary batteries can be used.However, graphitized carbon is preferable, and those having a specificsurface area of not more than 2.0 m²/g (preferably 0.5-1.5 m²/g) arepreferable, and those having a specific surface area of not more than2.0 m²/g (preferably 0.5-1.5 m²/g) and a spacing of lattice planes(d002) of not more than 0.3380 nm (preferably 0.3355-0.3370 nm) and acrystallite size in the c-axis direction (Lc) of not less than 30 nm(preferably 40-70 nm) are particularly preferable. When a graphitizedcarbon having a spacing of lattice planes (d002) of more than 0.3380 nmor having a crystallite size in the c-axis direction (Lc) of less than30 nm is used, the voltage of the negative electrode plate may beincreased to unpreferably decrease average discharge potential of thebattery.

[0091] The particles constituting the graphitized carbon are notparticularly limited in shape, and they can be scaly, fibrous,spherical, pseudo-spherical, bulky, whisker and the like. However, thegraphitized carbon is preferably fibrous.

[0092] When they are fibrous, those having an average fiber length of1-100 μm, particularly preferably 2-50 μm, specifically preferably 3-25μm, are preferable to improve coatability of an active materialcomposition to a collector. The average fiber diameter is preferably 0.5μm-15 μm, particularly preferably 1 μm-15 μm, specifically preferably 5μm-10 μm. In this event, the aspect ratio (average fiber length/averagefiber diameter ratio) is more preferably 1-5.

[0093] The fiber diameter and fiber length of the above-mentionedfibrous graphitized carbon are measured using an electron microscope.Specifically, the magnification is set to a value that contains at least20 fibers in the view and an electron microscopic photograph is taken.The fiber diameter and fiber length of each fiber on the photograph aremeasured with a caliper. The fiber length is the shortest distancebetween one end to the other end of a fiber when it is linear. When thefiber is curled, the fiber length is the distance between two optionaland most distant points on the fiber.

[0094] As the binder to be used along with the negative electrode activematerial, polytetrafluoroethylene, polyvinylidene fluoride,polyethylene, ethylene-propylene-diene type polymer and the like can beused as conventional.

[0095] The negative electrode active material layer may contain aconductive material as necessary. In this case, the conductive materialis exemplified by carbon blacks having an average particle size of notmore than 5 μm, and the like. As the collector to be used as thenegative electrode, conventional ones can be used, such as foil andexpanded metal made from copper, nickel, silver, stainless and the like.

[0096] The negative electrode active material layer is basically formedby a method (kneading step, painting step, drying step, roll-spreadingstep) similar to the methods for the aforementioned positive electrodeactive material layer, and the thickness thereof is preferably 50-300μm, particularly preferably 80-250 μm, specifically preferably 100-200μm. As used herein, by the “the thickness of the negative electrodeactive material layer” is meant the thickness of the negative electrodeactive material layer formed on one surface of a collector, when thenegative electrode active material layer is formed on one surfacethereof, and when the negative electrode active material layers areformed on both surfaces of the collector, the total of the thickness ofthe two negative electrode active material layers formed on the bothsurfaces thereof.

[0097] The lithium ion polymer secondary battery of the presentinvention is formed by disposing a solid electrolyte layer comprising aporous element comprising a fluoropolymer comprising vinylidene fluorideas a main unit, a salt and a compatible solvent between a positiveelectrode and a negative electrode and the form of the battery is notparticularly limited. For example, the following forms (i), (ii) and(iii) are preferable.

[0098] (i) A form prepared by intervening a belt-shaped solidelectrolyte layer between a belt-shaped positive electrode and abelt-shaped negative electrode and spirally winding them to give a roll,which is then housed in an exterior package. Here, the belt-shapedpositive electrode and the belt-shaped negative electrode are made byforming active material layer on both surfaces of a collector.

[0099] (ii) A form prepared by providing a rectangular positiveelectrode plate, a rectangular negative electrode plate and arectangular solid electrolyte layer having about the same size withthem, forming a laminate comprising one or more repeats of a unitcomprising a rectangular positive electrode plate, a rectangularnegative electrode plate and a rectangular solid electrolyte layersandwiched between the electrodes, and the laminate is housed in anexterior package. Here, a negative electrode plate forming an activematerial layer on one surface of a collector is disposed on theuppermost and the lowermost positions and other negative electrodeplates and the positive electrode plates comprise active material layerson both surfaces of a collector.

[0100] (iii) As shown in FIG. 3, a laminate structure is preparedwherein a belt-shaped solid electrolyte layer 12 is superimposed on arectangular negative electrode plate 11, on which a rectangular positiveelectrode plate 13 is superimposed, the negative electrode plate 11 anda positive electrode plate 13 are alternately laminated via a solidelectrolyte layer 12 while repeatedly folding the belt-shaped solidelectrolyte layer 12, and a positive electrode plate 13 is disposed onthe uppermost position, and the laminate structure is housed in anexterior package. Here, the lowermost and the uppermost negativeelectrode plates comprise an active material layer formed on one surfaceof a collector and other negative electrode plates and positiveelectrode plates have active material layers formed on both surfaces ofa collector.

[0101] As the above-mentioned exterior package, metal cans such as acylindrical tube can, a square tube can, a button-shaped can and thelike, a sheet exterior package such as a laminate film and the like areused. As a laminate film, a film comprising a laminate layer of athermoplastic resin such as polyester, polypropylene and the like isformed at least on one surface of a metal foil made of copper, aluminumand the like is preferable. A film having such thermoplastic resinlaminate layer can be sealed by armoring the film on the above-mentionedroll or laminate structure (stuck) and heat-welding the peripherythereof, facilitating the assembly of batteries.

[0102] In the case of the form of the above-mentioned (i), for example,a roll having a complete round (about complete round) section isprepared and housed in a cylindrical tube can to give a cylindrical tubetype battery. When a roll having a rectangular section is prepared andhoused in a square tube can, a square tube type battery is obtained.When a roll having a flat section (elliptic etc.) is prepared and abattery is composed by armoring with an exterior package sheet of alaminate film and the like, what is called, a sheet battery is obtained,which is thinner than can type batteries. On the other hand, the formsof the above-mentioned (ii) and (iii) are generally applied to sheetbatteries, in which case a laminate structure comprising a positiveelectrode plate, a negative electrode plate and a solid electrolytelayer is armored with an exterior package sheet made of a laminate filmand the like.

[0103] When a sheet battery is to be composed using a roll having a flatsection (i.e., the form of the above-mentioned (i)), a power generationelement (i.e., a structure containing a positive electrode, a negativeelectrode and a solid electrolyte layer) can be easily prepared andwinding enables laminating a belt-shaped positive electrode, abelt-shaped negative electrode and a solid electrolyte layer withsuperior adhesiveness. Thus, it is advantageous for producing thinnerbatteries. In contrast, when a sheet battery is to be composed using alaminate structure (i.e., the form of the above-mentioned (ii) or(iii)), the electrode does not include many useless parts and thebattery can be advantageously made to have high capacity.

[0104] In the lithium ion secondary battery of the present invention,when the form of the battery is the above-mentioned (i), wherein abelt-shaped solid electrolyte layer is disposed between the belt-shapedpositive electrode and the belt-shaped negative electrode and spirallywound to give a roll, which is housed in an exterior package, the rollis preferably has a constitution of the following (a) and/or (b).

[0105] (a) The total layer thickness A of positive electrode activematerial layers formed on both surfaces of a collector of a belt-shapedpositive electrode in the roll is set to 80-250 μm, total layerthickness B of negative electrode active material layers formed on bothsurfaces of a collector of the belt-shaped negative electrode is set to80-250 μm and the ratio (A/B) of the total layer thickness A to thetotal layer thickness B is set to 0.4-2.2. With this constitution,peeling off of an active material layer, occurrence of crack and thelike of the positive electrode and negative electrode can be preventedduring preparation of a roll, and the energy density of the battery canbe markedly increased, thereby achieving preferable results of high-ratedischarge characteristics.

[0106] (b) As shown in FIGS. 1 and 2, in a roll 100, an outermost rollpart 2 a of a belt-shaped negative electrode 2 is disposed on thefurther outer periphery of an outermost roll part 1 a of a belt-shapedpositive electrode 1; a first extrusion part 2A extruding from a freeend of an outermost roll part 1 a of a belt-shaped positive electrode 1is formed at a free end of an outermost roll part 2 a of a belt-shapednegative electrode 2; an innermost roll part 2 b of the belt-shapednegative electrode 2 is disposed on the further inner periphery of aninnermost roll part 1 b of the belt-shaped positive electrode 1; asecond extrusion part 2B extruding from a free end of the innermost rollpart 1 b of the belt-shaped positive electrode 1 is formed at a free endof the innermost roll part 2 b of the belt-shaped negative electrode 2;and a third and a fourth extrusion parts 2C-1 and 2C-2, respectively,are formed on both ends in the width direction of the belt-shapednegative electrode 2, which extrude from both ends in the widthdirection of the belt-shaped positive electrode 1. FIG. 1 is a crosssectional view of the roll wherein 3 therein shows a solid electrolytelayer. FIG. 2 shows the size and the positional relationship between thebelt-shaped positive electrode and the belt-shaped negative electrode,wherein the solid electrolyte layer is omitted.

[0107] With this constitution, the number of windings of the belt-shapednegative electrode becomes larger by once than that of the belt-shapedpositive electrode, and the parts extruding from the belt-shapedpositive electrode are comprised on the outermost periphery, theinnermost roll part and both ends in the width direction of thebelt-shaped negative electrode. As a result, the amount of lithium ion(lithium ion amount of negative electrode) to be doped in the negativeelectrode during charging can be increased. Particularly, the lithiumion capacity of the negative electrode at the end surfaces of the widthdirection and the longitudinal direction of the negative electrode,where lithium ion easily precipitate, can be increased, and morepreferable results in the high-rate discharge characteristics can beafforded.

[0108]FIGS. 1 and 2 show a roll for a sheet battery having a flatsectional shape. The same effect as mentioned above can be achieved witha roll for can type batteries having a complete round (about completeround) sectional shape, by forming the above-mentioned first to fourthextrusion parts on the belt-shaped negative electrode.

[0109] The size of the belt-shaped positive electrode (length L1, widthW1), the size of the belt-shaped negative electrode (length L2, widthW2), and extrusion lengths L3-L6 of the first to fourth extrusion partsof a roll for sheet batteries having a flat section are preferably setas in the following. L1: 550-650 mm, W1: 35-45 mm, L2: 620-720 mm, W2:37-47 mm, L3: 1-10 mm, L4: 1-10 mm, L5: 0.5-3 mm, L6: 0.5-3 mm.

[0110] The preferable size of the section of a roll having a flatsection is major axis of about 20-50 mm and minor axis of about 3-15 mm.

[0111] The size of the belt-shaped positive electrode (length, width),the size of the belt-shaped negative electrode (length, width), andextrusion lengths of the first to fourth extrusion parts of a roll forcan type batteries having a complete round (about complete round)section as defined using the symbols of the corresponding parts in FIGS.1 and 2 are preferably set, for example, as in the following.

[0112] L1: 550-650 mm, W1: 35-45 mm, L2: 620-720 mm, W2: 37-47 mm, L3:1-7 mm, L4: 1-7 mm, L5: 0.5-3 mm, L6: 0.5-3 mm.

[0113] The preferable size of the section of a roll having a completeround (about complete round) section is diameter of about 10-25 mm.

[0114] In the lithium ion polymer secondary battery of the presentinvention, as various constituent members not mentioned above such as alid of a battery can, a safety structure, an electrode terminal (leadterminal of sheet battery) and the like, existing items and products tobe developed in the future can be used.

[0115] The “density (apparent density)” of the porous element comprisinga fluoropolymer comprising vinylidene fluoride as a main unit in thepresent specification was obtained by cutting out 10 cm×10 cm samplesfrom a foamed (porous) film made of this polymer, measuring the volume(V) and weight (W) of the sample under the standard state according toJIS Z 8703 free of compression, and calculating by the following formula(VI). The thickness of the sample was obtained by precisely measuring at5 different points using a micrometer and taking an average.

apparent density (g/cm³)=W/V  (VI)

[0116] The “Gurley value” was measured according to the method describedin JIS P 8117 using round samples (specimens) having a diameter of 28.6mm, which were cut out from a foamed (porous) film made of this polymer.

EXAMPLES

[0117] The present invention is explained in more detail in thefollowing by referring to examples. The present invention is not limitedin any way by the following examples.

Example 1

[0118] [Preparation of Belt-shaped Positive Electrode]

[0119] A composition comprising LiCoO₂ (91 parts by weight, averageparticle size: 20 μm, 20/(average particle size [μm]×specific surfacearea [m²/g]): 7.8) as a positive electrode active material, sphericalartificial graphitized carbon (5 parts by weight, average particle size:6 μm, specific surface area: 3 m²/g) as a conductive material, oilfurnace black (1 part by weight, average particle size: 40 nm, specificsurface area: 700 m²/g) similarly as a conductive material, andpolyvinylidene fluoride (PVdF, 3 parts by weight) as a binder wasuniformly dispersed in N-methylpyrrolidone and kneaded to give a slurry.

[0120] The above-mentioned slurry was applied onto the both surfaces ofan aluminum foil (thickness 20 μm) to be a belt-shaped collector, dried,subjected to a roll-spreading treatment under the roll-spreadingconditions of roll-spreading temperature 30° C. and roll-spreading rate30% to form a positive electrode active material layer having athickness of 140 μm (total thickness of two positive electrode activematerial layers formed on the collector 140 μm), which was used as abelt-shaped positive electrode (width: 55 mm, length: 600 mm).

[0121] [Preparation of Belt-shaped Negative Electrode]

[0122] Fibrous graphitized carbon (95 parts by weight, specific surfacearea: 1.32 m²/g, spacing of lattice planes: 0.3364 nm, crystallite sizein the c-axis direction: 50 nm) to be a negative electrode activematerial, polyvinylidene fluoride (PVdF) (5 parts by weight) to be abinder, and N-methylpyrrolidone (50 parts by weight) were mixed to givea slurry. This slurry was applied onto the both surfaces of a copperfoil (thickness 1 μm) to be a belt-shaped collector, dried, subjected toa roll-spreading treatment under the roll-spreading conditions ofroll-spreading temperature 120° C. and roll-spreading rate 20% to give abelt-shaped positive electrode (width: 57 mm, length: 650 mm) having anegative electrode active material layer having a thickness of 150 μm(total thickness of two negative electrode active material layers formedon the collector 150 μm).

[0123] [Preparation of Porous Film for Solid Electrolyte Layer]

[0124] Polyvinylidene fluoride (PVdF, 30 parts by weight),dimethylformamide (DMF, 170 parts by weight) and n-octanol (35 parts byweight) as a foaming agent were mixed in a screw type blender to give acoating liquid (paste). The coating liquid (paste) was applied to asurface of an Al release substrate with a transcription type applicatorat application line speed 1 m/min. The coating film was dried by heating(6 min.) at 160° C. for evaporation of solvent and foaming. The obtainedfilm (thickness 25 μm) was peeled off from the release substrate. Thedensity of the film was 0.82 g/cm³, Gurley value was 35 sec/100 cc. Thisporous polyvinylidene fluoride (PVdF) film was cut into a belt having awidth of 59 mm and a length of 650 mm.

[0125] [Preparation of Lithium ion Secondary Battery]

[0126] The belt-shaped positive electrode, the belt-shaped negativeelectrode and the belt-shaped porous polyvinylidene fluoride (PVdF) filmprepared above were laminated in the order of the belt-shaped porouspolyvinylidene fluoride (PVdF) film, the belt-shaped negative electrode,the belt-shaped porous polyvinylidene fluoride (PVdF) film, and thebelt-shaped positive electrode and the laminate was wound with thebelt-shaped negative electrode placed inside, whereby a roll having aflat section (major axis of section 30 mm, minor axis of section 3.6 mm)having, at free ends of the innermost roll part and the outermost rollpart of a belt-shaped negative electrode, extrusion parts extruding by 5mm from the free ends of the outermost and innermost roll parts of thebelt-shaped positive electrode, and extrusion parts extruding by 1 mmfrom the both ends in the width direction of the belt-shaped positiveelectrode, at the both ends in the width direction of the belt-shapednegative electrode.

[0127] Then, the above-mentioned roll was immerse in an electrolyticsolution comprising LiPF₆ dissolved in a mixed solvent of ethylenecarbonate (20 vol %), propylene carbonate (10 vol %, ethylmethylcarbonate (60 vol %) and diethyl carbonate (10 vol %) in a concentrationof 1.0 mol/L (concentration after preparation) and a belt-shaped porouspolyvinylidene fluoride (PVdF) film sandwiched between the positiveelectrode and the negative electrode was impregnated with theelectrolytic solution to allow gellation. Then, this roll was housed inan Al laminate film obtained by laminating a thermoplastic resin, whichwas an exterior packaging material, on one surface to complete thebattery.

[0128] The impregnation with the electrolytic solution was performedsuch that the charge and discharge capacity obtained by actuallycharging and discharging the battery under the following conditionswould show 710 mA (predetermined charge and discharge capacity). Theamount of impregnation with the electrolytic solution then was 3.2 g.

[0129] [Charge and Discharge Conditions]

[0130] After charging with 350 mA current to reach 4.2 V, the currentwas flown for charging for the total of 4 hrs. while maintaining theconstant voltage, and then discharged with 350 mA current until 2.5 V.

Example 2

[0131] In the same manner as in Example 1 except that VdF-HFP(vinylidene fluoride-hexafluoropropylene copolymer) was used instead ofPVdF and the application line speed of the transcription type applicatorwas set to ⅔ of that in Example 1, namely, the heating time of thecoating film was changed to 9 min., a VdF-HFP porous film (thickness 23μm) having a density of 0.75 g/cm³ and a Gurley value of 43 sec/100 ccwas prepared. In the same manner as in Example 1 except that thisVdF-HFP porous film was used instead of the PVdF porous film, a lithiumion secondary battery was prepared. In the same manner as in Example 1,the impregnation amount of the electrolytic solution necessary forachieving the predetermined charge and discharge capacity was measuredand found to be 3.1 g. The average distance between the positiveelectrode and the negative electrode then was 28 μ(thickness of solidelectrolyte layers).

Example 3

[0132] In the same manner as in Example 1 except that the n-octanolcontent of the coating liquid (paste) was changed to 32 parts by weight,a PVdF porous film (thickness 24 μm) having a density of 0.81 g/cm³ anda Gurley value of 65 sec/100 cc was prepared. In the same manner as inExample 1 except that this PVdF porous film was used, a lithium ionsecondary battery was prepared. In the same manner as in Example 1, theimpregnation amount of the electrolytic solution necessary for achievingthe predetermined charge and discharge capacity was measured and foundto be 2.9 g.

Example 4

[0133] In the same manner as in Example 1 except that thedimethylformamide (DMF) content of the coating liquid (paste) waschanged to 150 parts by weight, a PVdF porous film (thickness 27 μm)having a density of 0.75 g/cm³ and a Gurley value of 125 sec/100 cc wasprepared. In the same manner as in Example 1 except that this PVdFporous film was used, a lithium ion secondary battery was prepared. Inthe same manner as in Example 1, the impregnation amount of theelectrolytic solution necessary for achieving the predetermined chargeand discharge capacity was measured and found to be 3.4 g.

Example 5

[0134] In the same manner as in Example 1 except that PVdF (6 parts byweight) and VdF-HFP (24 parts by weight) were used instead of PVdF (30parts by weight), a porous film of a mixture of PVdF and VdF-HFP(thickness 30 μm) having a density of 0.75 g/cm³ and a Gurley value of140 sec/100 cc was prepared. In the same manner as in Example 1 exceptthat this porous film of a mixture of PVdF and VdF-HFP was used, alithium ion secondary battery was prepared. In the same manner as inExample 1, the impregnation amount of the electrolytic solutionnecessary for achieving the predetermined charge and discharge capacitywas measured and found to be 3.1 g.

Example 6

[0135] In the same manner as in Example 1 except that the amount of PVdFwas changed to 25 parts by weight, a PVdF porous film (thickness 18 μm)having a density of 1.20 g/cm³ and a Gurley value of 45 sec/100 cc wasprepared. In the same manner as in Example 1 except that this PVdFporous film was used, a lithium ion secondary battery was prepared. Inthe same manner as in Example 1, the impregnation amount of theelectrolytic solution necessary for achieving the predetermined chargeand discharge capacity was measured and found to be 2.9 g.

Example 7

[0136] In the same manner as in Example 1 except that PVdF (30 parts byweight), DMF (230 parts by weight) and n-octanol (35 parts by weight)were mixed, and the application line speed was set to 1/5, namely,drying by heating for 30 min. was applied, a PVdF porous film (thickness30 μm) having a density of 0.60 g/cm³ and a Gurley value of 45 sec/100cc was prepared. In the same manner as in Example 1 except that thisPVdF porous film was used, a lithium ion secondary battery was prepared.In the same manner as in Example 1, the impregnation amount of theelectrolytic solution necessary for achieving the predetermined chargeand discharge capacity was measured and found to be 3.3 g.

Example 8

[0137] In the same manner as in Example 1 except that thedimethylformamide (DMF) content of the coating liquid (paste) was set to230 parts by weight and n-octanol (40 parts by weight) was used, a PVdFporous film (thickness 35 μm) having a density of 0.75 g/cm³ and aGurley value of 15 sec/100 cc was prepared. In the same manner as inExample 1 except that this PVdF porous film was used, a lithium ionsecondary battery was prepared. In the same manner as in Example 1, theimpregnation amount of the electrolytic solution necessary for achievingthe predetermined charge and discharge capacity was measured and foundto be 3.1 g.

Comparative Example 1

[0138] In the same manner as in Example 1 except that a polypropyleneporous separator (density 0.74 g/cm³, Gurley value 45 sec/100 cc) havinga thickness of 25 μm was used instead of the PVdF porous film, a lithiumion secondary battery was prepared. In the same manner as in Example 1,the impregnation amount of the electrolytic solution necessary forachieving the predetermined charge and discharge capacity was measuredand found to be 5.8 g.

Comparative Example 2

[0139] In the same manner as in Example 1 except that a PVdF porous film(thickness 24 μm) having a density of 1.4 g/cm³ and a Gurley value of 62sec/100 cc was prepared and this PVdF porous film was used, a lithiumion secondary battery was prepared. In the same manner as in Example 1,the impregnation amount of the electrolytic solution necessary forachieving the predetermined charge and discharge capacity was measuredand found to be 2.7 g.

Comparative Example 3

[0140] In the same manner as in Example 1 except that a PVdF porous film(thickness 26 μm) having a density of 0.54 g/cm³ and a Gurley value of 3sec/100 cc was prepared and this PVdF porous film was used, a lithiumion secondary battery was prepared. However, this battery was incapableof charge and discharge due to a short circuit.

Comparative Example 4

[0141] In the same manner as in Example 1 except that a PVdF porous film(thickness 24 μm) having a density of 0.77 g/cm³ and a Gurley value of180 sec/100 cc was prepared and this PVdF porous film was used, alithium ion secondary battery was prepared. In the same manner as inExample 1, the impregnation amount of the electrolytic solutionnecessary for achieving the predetermined charge and discharge capacitywas measured and found to be 3.4 g.

Comparative Example 5

[0142] In the same manner as in Example 1 except that a PVdF porous film(thickness 25 μm) having a density of 0.15 g/cm³ and a Gurley value of320 sec/100 cc was prepared and this PVdF porous film was used, alithium ion secondary battery was prepared. In the same manner as inExample 1, the impregnation amount of the electrolytic solutionnecessary for achieving the predetermined charge and discharge capacitywas measured and found to be 2.5 g.

[0143] The lithium ion secondary batteries of Examples 1-8 andComparative Examples 1-5 prepared above were subjected to the followingtests.

[0144] [Low Temperature Characteristics Test]

[0145] After charging at room temperature, the battery is left standingin an atmosphere at −20° C. for 24 hrs. The charging comprised passageof current at IC (600 mA) constant current until voltage reached 4.2 V,followed by passage of current at 4.2 V constant voltage for the totalcharge time of 2.5 hrs. Then, discharge is conducted at 0.5C (300mAh)/2.5 V cut off voltage in this atmosphere at −20° C., and thedischarge capacity [mA·H] at this time is determined. In addition,charge and discharge are also conducted under the similar conditions atroom temperature (20° C.) and discharge capacity [mA·H] is determined.Further, the discharge capacity at −20° C. was divided by the dischargecapacity at room temperature to determine changes in the dischargecapacity.

[0146] [High-rate Characteristics Test]

[0147] 2C Discharge was conducted at room temperature (20° C.) and theproportion of the discharge capacity relative to the total capacity wascalculated. By the 2C is meant the constant current at 1200 mA relativeto the discharge capacity (600 mA) of lithium ion secondary battery.

[0148] [Cycle Characteristics Test]

[0149] 1C/1C Charge and discharge (500 cycles) are conducted at roomtemperature (20° C.) and discharge capacity [mA·H] is calculated fromdischarge current and discharge time for 1 cycle and 500 cycles. Then,discharge capacity [mA·H] at 500 cycles was divided by the dischargecapacity [mA·H] at 1 cycle to determine changes [%] in the dischargecapacity.

[0150] These results are shown in the following Tables 1 and 2. TABLE 1Examples 1 2 3 4 5 6 7 8 polymer substrate of solid PVdF VdF-HFP PVdFPVdF PVdF&VdF-HFP PVdF PVdF PVdF electrolyte layer density (g/cm³) ofporous film 0.82 0.75 0.81 0.75 0.75 1.20 0.60 0.75 Gurley value(sec/100 cc) of 35 43 65 125 140 45 45 15 porous film amount (g) ofelectrolytic 3.2 3.1 2.9 3.4 3.1 2.9 3.3 3.1 solution to obtainpredetermined charge and discharge capacity cycle characteristics (%) 7978 76 77 72 75 76 70 low temperature 81 80 75 74 80 79 79 83characteristics (−20° C., 1C) (%) high-rate characteristics 95 97 96 9596 95 94 97 (2C) (%)

[0151] TABLE 2 Comparative Examples 1 2 3 4 5 polymer substrate of PEPVdF PVdF PVdF PVdF solid electrolyte layer (separator) density (g/cm³)of 0.74 1.40 0.54 0.77 0.15 porous film Gurley value (sec/100 cc) 45 623 180 320 of porous film amount (g) of electrolytic 5.8 2.7 Discharge3.4 2.5 solution to obtain unavailable predetermined charge due to anddischarge short capacity circuit cycle characteristics (%) 45 52 70 30low temperature 48 12 15 Un- characteristics known (−20° C., 1C) (%)high-rate characteristics 73 70 80 38 (2C) (%)

[0152] From Tables 1 and 2, it is evident that Examples 1-8 wherein aporous element made of a fluoropolymer comprising vinylidene fluoride asa main unit, which has a density and Gurley value within the rangesdefined in the present invention, was used as a polymer substrate of asolid electrolyte layer showed superior cycle characteristics, lowtemperature characteristics and high-rate discharge characteristics, ascompared to Comparative Examples 2-4 wherein a porous element made of afluoropolymer comprising vinylidene fluoride as a main unit, which hadat least one of a density and a Gurley value outside the range definedin the present invention.

Industrial Applicability

[0153] As is clear from the foregoing explanation, according to thepresent invention, by the use of a porous element made of afluoropolymer comprising vinylidene fluoride as a main unit, which has aparticular density and a particular Gurley value as a polymer substrateof a solid electrolyte layer, a lithium ion polymer secondary batteryhaving strikingly improved low temperature characteristics, cyclecharacteristics and high-rate discharge characteristics as compared toconventional batteries can be obtained.

[0154] This application is based on patent application Nos. 395543/2000and 386624/2001 filed in Japan, the contents of which are herebyincorporated by reference.

What is claimed is:
 1. A lithium ion polymer secondary batterycomprising a positive electrode, a negative electrode and a solidelectrolyte layer comprising a porous element comprising a fluoropolymercomprising vinylidene fluoride as a main unit and having a density of0.55-1.30 g/cm³ and a Gurley value of not more than 150 sec/100 cc, asalt and a compatible solvent, which is disposed between the positiveelectrode and the negative electrode.
 2. The lithium ion polymersecondary battery of claim 1, wherein the salt is at least one kind ofcompound selected from LiClO₄, LiBF₄, LiPF₆, LiAsF₆, LiCF₃SO₃, LiAlCl₄and Li(CF₃SO₂)₂N, and the compatible solvent is a mixed solvent of oneor more kinds selected from ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, dimethylsulfoxide, sulfolane, γ-butyrolactone, 1,2-dimethoxyethane,N,N-dimethylformamide, tetrahydrofuran, 1,3-dioxolane,2-methyltetrahydrofuran and diethyl ether.
 3. The lithium ion polymersecondary battery of claim 1 or 2, wherein the positive electrode activematerial is a Li-transition metal composite oxide.
 4. The lithium ionpolymer secondary battery of any of claims 1 to 3, wherein the negativeelectrode active material is a graphite.
 5. The lithium ion polymersecondary battery of any of claims 1 to 4, wherein the positiveelectrode is a belt-shaped positive electrode comprising positiveelectrode active material layers, which comprise an active material anda conductive material, formed on both surfaces of a belt-shapedcollector, the negative electrode is a belt-shaped negative electrodecomprising negative electrode active material layers formed on bothsurfaces of a belt-shaped collector, and these belt-shaped positiveelectrode and belt-shaped negative electrode and the solid electrolytelayer having a belt shape and being interposed between said electrodesare spirally wound to constitute a roll, wherein the total thickness Aof the positive electrode active material layers formed on both surfacesof the belt-shaped collector of said belt-shaped positive electrode andthe total thickness B of the negative electrode active material layersformed on both surfaces of the belt-shaped collector of said belt-shapednegative electrode are each 80 μm-250 μm, and the ratio (A/B) of thetotal thickness A to the total thickness B is 0.4-2.2.
 6. The lithiumion polymer secondary battery of any of claims 1 to 4, wherein thepositive electrode is a belt-shaped positive electrode comprisingpositive electrode active material layers, which comprise an activematerial and a conductive material, formed on both surfaces of abelt-shaped collector, the negative electrode is a belt-shaped negativeelectrode comprising negative electrode active material layers formed onboth surfaces of a belt-shaped collector, and these belt-shaped positiveelectrode and belt-shaped negative electrode and the solid electrolytelayer having a belt shape and being interposed between the electrodesare spirally wound to constitute a roll, wherein an outermost roll partof said belt-shaped negative electrode is disposed on a still outerperiphery of the outermost roll part of said belt-shaped positiveelectrode, and a first extrusion part extruding from a free end of theoutermost roll part of said belt-shaped positive electrode is formed ona free end of the outermost roll part of said belt-shaped negativeelectrode, an innermost roll part of said belt-shaped negative electrodeis disposed on a still inner periphery of the innermost roll part ofsaid belt-shaped positive electrode and a second extrusion partextruding from a free end of the innermost roll part of said belt-shapedpositive electrode is formed on a free end of the innermost roll part ofsaid belt-shaped negative electrode, and a third and a fourth extrusionparts extruding from both ends in the width direction of saidbelt-shaped positive electrode are respectively formed on both ends inthe width direction of said belt-shaped negative electrode.